Using the trackpad gives a finer magnification compared to keyboard/mousewheel

[bathienle]

To fix by having finer magnification with the keyboard/mousewheel as well.

[urubens]

To keep consistency with the rest of the implementation, I think it's better to refer here as zoom instead of magnification, even though it describes the same concepts. In Cytomine implementation, we nearly often refer as zoom, an number describing the tier in the image pyramid, 0 being the top of the pyramid. Magnification is more a end-user information: we save the slide magnification at best resolution (pyramid basis) and compute it on the fly using the zoom value.

[bathienle]

Okay, I will refactor the code to refer as zoom from now on.